The present invention relates to a flowmeter, and more particularly to a flowmeter for measuring the flow rate of a fluid by using a vibrator positioned in the fluid to be able to vibrate.
Vortex flowmeters and Differential Producing flowmeters have been heretofore used to measure the flow rate of a fluid.
The vortex flowmeter is based on the principle that, as shown in FIG. 10, if an object 54 is disposed in a flow passage 53 of a fluid, alternate vortices (von Karman's vortices) are generated in two lines along the flow passage on the rear side of the object 54. The flow rate of the fluid is measured by counting the number of the yon Karman's vortices.
Specifically, the vortex flowmeter comprises a vortex generating mechanism and a mechanism for detecting vortices generated by the vortex generating mechanism. The number of vortices generated by the vortex generating mechanism in a fluid is detected by a thermistor or a piezoelectric transducer, thereby measuring the flow rate of the fluid.
In such a conventional vortex flowmeter, however, because frequency of the fluid is detected by generating the von Karman's vortices 55, the vortex generating mechanism having a flow passage of width not less than a certain value, i.e., having a predetermined size, is required in order to generate the yon Karman's vortices 55. Therefore, the vortex flowmeter cannot have a size smaller than a certain limit. Also, when the flow velocity or flow rate of the fluid is small, the flow rate cannot be measured since it is difficult to generate the yon Karman's vortices 55 under such a condition.
Meanwhile, the so-called Differential Producing flowmeters have also been conventionally used in wide fields. The Differential Producing flowmeter includes a throttling mechanism such as an orifice or venturi in a flow passage. The flow rate of a fluid is measured based on a differential pressure produced across the throttling mechanism.
However, such a conventional Differential Producing flowmeter has had shortcomings below. A differential pressure transmitter is required to measure the differential pressure. Further, since the measured flow rate is expressed in the form of a quadratic curve, a flow rate conversion unit including a calculation circuit is required to convert the measured flow rate into a linear value for easier use in subsequent steps. As a result, the production cost is increased.
Also, in the conventional Differential Producing flowmeter, when fluid conditions such as density, pressure and temperature are varied depending on the kind of fluid to be measured, calculations for compensating measured values are required, making it troublesome to perform the measurement work.